The present invention concerns a process for drying a paper web in a paper making machine and a machine drying section which practices the process.
A large number of publications are concerned with paper drying technology. Refer, for example, to U.S. application Ser. No. 07/931,261 filed Aug. 17, 1992 or the German equivalent, GM 92 07 656. This application involves a drying section for a paper making machine with several drying groups arranged in sequence along the web path through the drying section. Each individual drying group has a single row of drying cylinders. This single row of drying cylinders is called a single tier drying group. Between each two neighboring drying cylinders in a row there is a web transfer roll in the form of a suction guide roll for the web. A respective single endless web support belt or dryer screen or dryer fabric extends through each drying group and passes around a drying cylinder and then a suction guide roller alternately, along a meandering pathway. The paper web to be dried is carried on the one side of the support belt selected so that the web is in direct contact with the drying cylinders in the drying group while the support belt is in direct contact with the alternate suction guide rollers. As a result, the paper web is dried by being heated from only one side in a single tier drying group. In order to dry the web by heating it from its other side, another drying group follows the first mentioned group, and if the following group is a single tier drying group, the paper web is supported on the side of the respective support belt that the web makes contact with the drying cylinders on the other side of the web.
As an alternative to single tier drying groups, there are drying groups in which the paper web alternately goes around an upper row drying cylinder and then around a lower row drying cylinder, so that in moving from cylinder to cylinder along the web path within one drying group, the two sides of the paper web alternately are in direct contact with a heated drying cylinder. This type of drying group is known as a double tier group.
The present invention is suitable for both single tier and double tier drying groups in a drying section. There are several requirements for a paper web drying section. The web should pass through the drying section with great security, even at the highest working velocities. As far as possible, web breaks should be avoided for good runability. Web drying should be as intensive as possible, so that a certain amount of water contained in the paper web should be driven out over the shortest possible distance along the web path, which enables the length of the drying section when constructed to be kept as small as possible. This provides high energy density. In addition to these properties, there are other requirements for the drying section and the drying process. In many types of paper, gentle drying is very important in order to influence certain paper properties. A primary objective is to avoid wrinkling.
The present invention directs special attention to providing the highest possible energy density and thereby to provide rapid and intensive drying. One known means of removing large amounts of moisture from a paper web over a short web path consists of increasing the temperature of the peripheral coverings of the individual drying cylinders. There are limits on this, however, involving for example the strength of the material of the drying cylinder. In addition to the energy density requirement, attention should also be paid to the requirement for gentle drying.